This present invention is directed generally to hologram fabricating techniques, and is directed more particularly to a technique for controlling effective diffraction efficiency in holograms.
In the manufacture of holograms such as those utilized for vehicle head-up display systems, it may be desirable to closely control hologram efficiency and bandwidth within narrow limits to meet the required optical performance. This is due to the interrelationships between parameters including diffraction efficiency, see-through and angular bandwidth.
However, reproducibility of holograms having the desired efficiency and bandwidth is difficult due to the extreme sensitivity of hologram performance to a variety of factors in the recording process. Some of these factors include:
1. Recording material: age, storage conditions, pre-exposure hardening, molecular composition and moisture content.
2. Chemical processing: temperature, time, agitation, chemical concentration and subsequent drying conditions.
3. Laser exposure: beam power, time, coherence length, beam ratio and mechanical stability. With present techniques, it is difficult to precisely control all of these parameters, and manufacturing yield tends to be quite low.
Present techniques to meet tight diffraction efficiency requirements are directed to precisely controlling the hologram manufacturing parameters. However, such tight control is extremely difficult to achieve with some materials such as dichromated gelatin, wherein recording involves molecular crosslinking that is non-linearly affected by such parameters as the organic molecular structure of animal protein, temperature, moisture content, laser exposure energy, gelatin rate during film coating, previous history of the gelatin molecules, rate of dehydration after wet processing, swelling of the gelatin during processing, time and temperature of baking after processing, and others.
Presently, the recording medium thickness is selected to provide the desired bandwidth, then all other fabrication parameters are selected and controlled to give the desired diffraction efficiency. In particular, a diffraction efficiency vs. effective exposure curve indicates the relationship of diffraction efficiency to effective exposure, wherein the effective exposure level includes all the variable in recording material, laser exposure and chemical processing. In view of the number of exposure variables and their non-linearity and sensitivity to many factors, attempting to control diffraction efficiency by tightly controlling exposure parameters is exceedingly difficult.